In a biological organ having a luminal structure such as blood vessels, the trachea and the intestines, when stenosis occurs therein, a cylinder-shaped flexible stent with mesh pattern is used in order to secure patency at a site of pathology by expanding an inner cavity at a narrowed part. These biological organs often have bent or tapered structures locally (i.e. a tubular structure of which sectional diameters of the inner cavity differ locally in an axial direction). Therefore, a stent having higher conformability has been desired which can flexibly adapt to such a complex vessel structure. Furthermore, in recent years, stents have come to also be employed for the treatment of cerebral blood vessels. Among tubular organs in a living body, the cerebral vessel system has a more complex structure. The cerebral vessel system has many bent sites and sites having tapered structures. Therefore, stents with particularly higher conformability have been required therein.
The structures of a stent generally include open cell structures and closed cell structures. In a stent having an open cell structure, an unconnected cell forms a strut having a free end. In a stent having a closed cell structure, every cell is connected and thus there is no strut having a free end.
Generally speaking, a stent having an open cell structure has a high conformability as compared to a stent having a closed cell structure, and thus the stent is suitable for placing in a tortuous tubular organ. Therefore, the stent is recognized as one having a stent structure which exerts remarkable mechanical flexibility in the axial direction. However, as illustrated in FIG. 13, in the stent 111 having an open cell structure, when bending and placing the stent 111 at a bent portion, a portion of the strut 117 easily protrudes radially outward from the stent 111 in a flared shape (refer to the portion surrounded by the dashed line of FIG. 13), a result of which there is a risk of damaging the tissue of a tubular organ in a body such as blood vessels. Furthermore, in particular, in bent blood vessels, when the strut 117 of the stent 111 located inside the blood vessel enters into a space located radially inside the stent 111, there is a risk of inhibiting blood flow and causing thrombus (refer to a portion surrounded by a short-long-dashed line of FIG. 13).
Furthermore, since the strut 117 protrudes in the stent 111 having the open cell structure, adhesion to the blood vessel wall BV (illustrated by a short-short-long-dashed line in FIGS. 13 and 14) is deteriorated in the bent blood vessel. Due to this, a space is generated between the stent and the blood vessel wall BV and thus there is a risk of causing thrombus herein. Moreover, since the adhesion to the blood vessel wall BV is deteriorated, stress concentration to the blood vessel wall BV results as illustrated in FIG. 14. Due to the stress concentration to the blood vessel wall BV by the stent 111, there is a risk of damaging the blood vessel wall BV since load is applied locally on the blood vessel wall BV. Moreover, at the portion to which stress concentration is applied, the risk of forming an inner membrane in excess in a blood vessel deformed by the stent 111 occurs, and thus lowers shear stress of the wall face which promotes regeneration of the membrane.
It should be noted that the two kinds of mechanical flexibilities in an axial direction (an axial direction, a central axial direction) and a radial direction (a vertical direction with respect to the axial direction) of the stent are said to be important for the purpose of realizing a stent with higher conformability. Herein, the flexibility in the axial direction refers to stiffness with respect to bending along the axial direction or the ease of bending, and thus is a property that is necessary for a stent to be flexibly bent along the axial direction so as to allow the stent to conform to a bent site of a tubular organ in a body. On the other hand, the flexibility in the radial direction refers to stiffness with respect to expansion and contraction in a vertical direction with respect to the axial direction or the ease of expansion and contraction, and thus is a property that is necessary for making the radius of a stent flexibly differ following the shape of an outer wall of a luminal structure of a tubular organ in a body so that the stent is in tight contact with the outer wall of the luminal structure.
In addition, regarding a stent, it is also an object to suppress shortening (refer to Japanese Unexamined Patent Application, Publication No. 2010-233933). When a stent mounted to a catheter in a state of being radially reduced is deployed (expanded) within a blood vessel during operation, the total length of the stent is shortened in the axial direction more than when crimped (radially reduced). The matter of the stent becoming shorter in the axial direction when expanding the stent, which is radially reduced, in such a way is referred to as “shortening”. The cause of shortening is as follows. As illustrated in FIG. 15, when expanding the stent which is radially reduced, the angle of the apex 172 made by the leg portions 171 in the cell 117 which is directed in the axial direction LD becomes greater (θ11<θ12). It should be noted that the reference line CL is a line running parallel with the axial direction LD and passes through the apex 172.
Along with this, since a circular body 113 having the cell 117 expands in a circumferential direction, the whole stent 111 is shortened in the axial direction LD. In particular, for a stent having an open cell structure, since it is difficult to store the stent again in the catheter, the stent is required to be placed precisely in a single operation. However, such shortening increases the degree of difficulty of the stent treatment for a medical doctor.    Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2010-233933